In the continuing effort to provide more efficient and fossil fuel free modes of propulsion, systems have been developed which utilize recovery and reuse of kinetic energy, such as that present in moving vehicles. One such system is known as regenerative braking (“regen”), wherein deceleration forces are captured and converted to a storable medium and then reused for the next acceleration event. The most popular and most exploited version of this technology is electrical regen. Electrical regen is used in virtually all “battery-only” electric vehicles, and in most “all-hybrid” vehicles. However, as with any technology, there are restrictions which limit the effectiveness and efficiency of the process.
For electrical regen systems, the storage medium is almost exclusively batteries, and in some rare cases, ultra-capacitors. Battery based regen systems are limited in their efficiency and storage capacity by chemical restrictions which prevent more than an average of twenty percent (20%) of the recoverable braking energy to be stored for reuse. In terms of the one hundred percent (100%) recoverable kinetic energy available for capture, storage and reuse, this chemical limitation results in roughly eighty percent (80%) of the remaining recoverable energy to be lost forever as heat in the friction braking system.
As a rule, batteries are either power dense, meaning a high amount of power per volume unit, or energy dense, referring to a high amount of energy per volume unit. Rare and expensive exceptions do exist that combine power density and energy density. However, their use in motive applications are very limited in scope, and these batteries are more generally used in stationary applications such as for grid-level electrical energy storage. Also, because batteries are either power or energy dense, their use in electric and hybrid vehicles suffer from operational functionalities which reduce their overall effectiveness, and thus reduce their range and/or performance at a given acceptable cost. Therefore, a more efficient means that electrical regen is needed for the recovering, storing and reusing of recoverable decelerative energy.
In an effort to provide a more effective and efficient regen system, hydraulic regen has gained attention due to its low cost, robust performance, durability and high efficiency. Hydraulic regen utilizes existing, market proven hydraulic components to perform the same function as electrical regen, wherein hydraulic accumulators are used as the “battery,” and hydraulic pumps and motors are used as the electrical motor and generator. Hydraulic regen systems provide benefits not available in electrical regen systems.
However, even hydraulic regen systems have drawbacks which limit overall performance and effectiveness. Hydraulic accumulators also have restrictive aspects in their operational parameters as a regenerative braking system, most importantly their lack of compact energy density. Hydraulic accumulators are restricted by volume, wherein a given volume of hydraulic fluid will provide a limited amount of acceleration before the fluid is exhausted and meaningful assistance ends. Academic research, prototype development and commercialized hydraulic regen only systems have proven to capture roughly eighty percent of the recoverable braking energy in a decelerating fossil fueled powered vehicle. In order for an accumulator based recovery system to have meaningful energy density, prohibitively large volumes must be utilized on board a vehicle. With space being at a premium in vehicular design, and weight being another important design factor, use of hydraulic regenerative systems is limited to specific applications, such as off road construction equipment, rail based transport, larger on road vehicles, and other similar uses.
In order to provide a balanced solution to the individual power density and energy density issues associated with each type of technology, and therefore maximize the efficiency and performance of the regen process, an electric/hydraulic hybrid regen system was developed utilizing the best positive attributes of both electrical and hydraulic regen systems while simultaneously eliminating the negative operational limitations of each individual technology. Such a combined dual regenerative drive system is disclosed in U.S. Pat. No. 7,201,095 (“'095 Patent”), incorporated herein by reference in its entirety for all purposes.
The '095 patent discloses both electrical and hydraulic regen, allowing for an onboard power dense hydraulic accumulator storage medium and an energy dense electrical storage medium. Combining these two technologies provides an optimized balance of power and energy densities utilizing specific electrical and hydraulic components. The '095 patent further discloses that deceleration energy is captured by at least one first hydraulic pump/motor through a rotational connection to at least one drive wheel. The first hydraulic pump provides the ability to move fluid through a second hydraulic pump/motor via fluidic connection to operate a rotationally connected electrical motor/generator which produces electrical energy to charge at least one battery, thereby providing hydraulically driven electrical regen. To maximize the efficiency and optimize the rate of charge to the battery, the torque and speed of the electrical generator is adjustable by utilizing at least one variable displacement hydraulic pump/motor so the input rotational speed of the first hydraulic pump/motor is independent of the output speed of the second hydraulic pump/motor.
Additionally, the '095 patent discloses the ability to bifurcate the input energy of the system into electrical and hydraulic storage. Therefore, the fluid may be routed in such a fashion as to fill at least a portion of at least one high pressure hydraulic accumulator by drawing fluid from at least one low pressure hydraulic fluid reservoir and directing said fluid to the high pressure hydraulic accumulator. The filling of the hydraulic accumulator can occur independent of or in conjunction with the electrical regenerative function via specific routing of the hydraulic fluid as described in the '095 patent.
Further, the '095 patent describes an operational modality of dual stored energy use that provides a stored hydraulic power launch and an electrically driven hydraulic drive wherein the short duration but high power density of the hydraulic accumulator is best suited to provide the power required to break inertia and launch a vehicle from a standstill and/or provide acceleration of a slowly moving vehicle while eliminating any electrical energy discharge from the battery. As the hydraulic accumulators volume is depleting and thus the pressure is falling, the electrically driven variable hydraulic drive system is utilized to either continue the vehicles acceleration and/or maintains the vehicles speed depending on the required operational input.
Although the '095 patent teaches a specific primary hydraulic input/output architecture to achieve the desired results of dual regenerative braking and dual power use, the design has application limitations that need to be addressed and remedied.
Therefore, new systems for improved recovery of kinetic energy are desirable, and in particular, alternative architectures are needed which afford more applications utilizing the synergy of combined dual use hydraulic and electric technologies.